If you set vpopmail up with mysql support its quite easy to get a full listing of domains/users. You'l only have to use the vpopmail php functions to add/delete users/domains in a own written front end.

This library is kind of useless without key functions like get_domain_info() and list_all_domains(). You can add and delete virtual domains, but you cannot list or view them. Without such functionality you can't go and write your own PHP front end to qmail.